The present invention relates generally to video display devices of the type for displaying characters and, more particularly, to an improved video amplifier arrangement for use therein with wideband variable D-C gain or level control providing an essentially flat response across the operating range.
In video display devices of the type for displaying character information in the form of letters and numbers, the video information is essentially in pulse form. That is, the video signal translated through the processing circuitry, including the video amplifier stages, is either all black or all white (full on or full off) to form the dots which are arranged in the desired character patterns on display.
The digital video signal information representing the characters to be displayed is in the form of pulse trains, some long, some relatively short. It will be appreciated that such pulse train information is particularly susceptible to deterioration and degradation if transmitted over substantial lengths of transmission cabling. Rise times of the individual pulses may be as sharp as five nanoseconds. This translates to around 50 MHz in terms of frequency response. It will be further appreciated that the amplitude level of the respective pulses in a pulse train will vary thereby causing a variation in brightness level regarding the displayed characters on the face of the associated cathode ray screen.
In addition, the control of the video level in such devices has, in the past, utilized conventional potentiometer across which the normalized video signal is applied. The distributed capacitance between the center arm and the resistance element varies with the setting. Accordingly, frequency response is not uniform, and there is, in fact, a high frequency roll-off, which may severely affect picture presentation.